This invention relates to a method for the measurement of the absolute differential delay between two or more associated signal streams (or digital bitstreams). One of the signals (or bitstreams), known as the primary signal, could represent picture or video content. The other or others would normally carry information related to, and time critical with, the primary channel. Examples of these secondary signals would be the audio or other picture related data, where the primary channel carried video information. The primary channel remains unaffected by the application of this invention whereas one or more secondary channels have a non-invasive timing reference signal added. This timing reference signal may be removed if desired.
It is well known for different signals (or bitstreams) to suffer from different delays, even when they are carried in the form of a combined channel or digital bitstream, or digital packet. This situation can arise, for instance, because of the different delays that are incurred by the different processing and coding procedures that are appropriate for the different types of signal content carried in the primary and secondary channels.
In particular cases where the primary signal carries video information, and the secondary channels carry audio associated with the video content, it is well known that there exists a restricted range over which the differential time delays are perceptually acceptable to human viewers. It is also known that correction of these delays by operator perception alone cannot be relied on to achieve the desirable close limits, especially where the delays concerned vary over a period of time.
A suitable timing measurement is described in UK Patent GB 2244184, where a system for the measurement of delay between a video signal and a single audio signal is described. This includes a tone burst generator that can be used to mark the audio signal at a single known period after a picture cut occurs in the video signal. The method has the advantage that the primary signal requires no intervention. However, the isolated marker signal inserted into the audio may be prone to interference from or confusion with the coexisting signal content, or by corruption which may take place in the coding, processing or decoding processes between the source and the destination signals. The result is that reliability of the marker detection may be poor under some circumstances, and the accuracy of the measurement will be limited by the single occurrence of the marker.
Other methods, such as that described in GB 2181325A, involve addition of a crude representation of the correctly timed secondary signal on to the primary signal. In this particular case, the addition is made to the vertical interval of an analogue video primary signal. Information placed in the vertical interval period is especially prone to removal or corruption during processing or standards conversion of the video signal.
According to the invention, there is provided a system for the measurement of the differential delay between a primary signal and a secondary signal or signals, propagating between a nominal source and a nominal destination, the system comprising, at said source:
first detection means detecting of a rapid change in a given parameter of said primary signal, and
applying means for applying a timing signal containing timing information relating to the rapid change in said parameter of said primary signal detected by said detection means to the secondary signal(s)
and at said destination:
second detection means for the detection of a rapid change in the said given parameter of the primary signal,
recovery means for recovering said timing signal from said secondary signal(s) and
third detection means for detection of the timing information relating to said rapid changes in the said parameter of said primary signal from the recovered timing signal.
Thus, in a preferred embodiment of the invention, a continuous or semi continuous time reference signal is utillised, which may be concealed by carriage in the form of a differentially modulated signal between two or more closely related secondary signals, for example, between two or more channels of stereo audio. The time reference signal which is carried may refer to an absolute reference time or it may carry a reference to the time datum at which a given measurable disturbance in a parameter of the primary signal occurs. When another measurable disturbance occurs in the primary signal, and allowing for a suitable delay for the first measurement to be completed, this time datum may be reset. The disturbance used as the time datum may be related to a portion or the whole of the primary signal content, or it may relate to a suitable system characteristic such as a data packet boundary. In either case, the associated secondary marker signal only requires a short period of correct reception in order for the time reference to be obtained. This period is free to occur singly or multiply at any time during the transmission period of the reference signal. Confirmation of correct reception may be obtained with the value obtained from later time reference reception periods.
Additionally, the time datum measurements may be applied, if necessary via an algorithm that confirms the validity of the values received, to variable delay apparatus in either primary or secondary signal paths. This will enable a correct timing relationship to be established. The timing changes necessary may be applied via a second algorithm which changes the necessary delay periods over a period of time in such a way that the signal content carried on the primary or secondary channels is not adversely affected by the delay changes.